Update: "We are looking into it at the moment," an Ubisoft representative has told us about the later vanishings outside the UK. Meanwhile, two dear readers in Finland and Germany comment that the games have disappeared from Steam for them too.
When Ubisoft's Assassin's Creed Unity, The Crew, and Far Cry 4 all vanished from Steam in the UK earlier this week, the publisher made cryptic mention of being "in discussions with Valve". What's so weird or special about the UK that our Steam wouldn't get Ubisoft's big fancy Christmas lineup? Turns out, we're not so special any more. These games are vanishing from the US and Australian stores too, VG247 noted, suggesting something bigger than a regional quirk is afoot.
Exactly what that's going on is a mystery at this point. I'm getting flashbacks to when EA started pulling games from Steam and went Origin-only, but that's only speculation.
Ubisoft's vague story yesterday was that they were chatting with Valve about something, but that it might only be a temporary vanishing.
"We've been in discussions with Valve about Assassin's Creed Unity but for the time being the game is not available via Steam in the UK," they told PCGamesN. "In the meantime, UK customers wishing to purchase the game digitally can do so by visiting the Uplay store, our retail partners or other digital distributors."
We've shot Ubisoft a line to see what they say now. To half-blindly draw comparisons, the problem EA had with Steam was wanting to "establish an ongoing relationship" with players, which they say was about providing updates and services themselves but rumour says was also about selling DLC directly without going through Valve - and Valve wouldn't allow that. I do hope we won't end up with Ubi's new games only available in the Uplay client and store; it's simply not as good as Steam.
For several years, Ubisoft games on Steam have required a Uplay account and launched the client anyway, but a few recent Uplay updates have deepened the connection. One in September let folks link Steam and Uplay accounts, "in order to seamlessly start your games from Steam." Another this very week (as LewieP points out) makes it so launching a Uplay game through Steam will show all your other Steam-bought Uplay games in the Uplay client, "making it easier to find the content you own." Which seems a bit like making it yell "Hey! Over here! Look! You can use this thing! It has games!"
The most recent update also added features one would expect from a client Ubisoft would want people to have open all the time, like minimising to the task bar tray and background autopatching (which can't be disabled). They're sensible things to add anyway, of course, but Ubisoft haven't seemed too serious before about making Uplay usable or desirable. Perhaps they are shoring it up to try ditching Steam and going their own way.
But this is, of course, all speculation and line-drawing.